Solar flares are dramatic flashes of brightness from the Sun that can throw large quantities of plasma through the solar corona and into outer space. They can be dangerous and there’s much we still don’t know about them. New observations, however, have taken us for the first time into the very heart of these flares. It's the first time the "central engine" of a large solar flare has been measured.

Researchers reporting in Nature Astronomy describe a close examination of a significant solar flare connected to a powerful eruption that happened in September 2017. They discovered that in the core flaring region, where magnetic field lines interact and reconnect, there is a huge electric current “sheet”. This region is 40,000 kilometers (25,000 miles) across and it is believed to be crucial to the acceleration of electrons into powerful eruptions.

"How exactly [the acceleration] happens is not clearly understood, but it is thought to be related to the Sun's magnetic field." lead author professor Bin Chen, from the New Jersey Institute of Technology, said in a statement. “It has long been suggested that the sudden release of magnetic energy through the reconnection current sheet is responsible for these major eruptions, yet there has been no measurement of its magnetic properties.

"With this study, we’ve finally measured the details of the magnetic field of a current sheet for the first time, giving us a new understanding of the central engine of the Sun’s solar flares."

The team used a combination of extreme ultraviolet emissions recorded by the Solar Dynamic Observatory and microwave observations from the Owens Valley Solar Array (EOVSA). Together, these observations suggest that the flare’s high-energy electrons are trapped and accelerated by a bottle-like magnetic structure 20,000 kilometers (12,500 miles) over the surface of the Sun.

"We found that there were a lot of accelerated particles just above the bright, flaring loops," said co-author Kathy Reeves. "The microwaves, coupled with modeling, tells us there is a minimum in the magnetic field at the location where we see the most accelerated particles, and a strong magnetic field in the linear, sheet-like structure further above the loops."

The study combined both observations and simulations, the latter of which helped in understanding how and where energy is stored and released in solar flares. These new observations provide precious new data on how one of the Sun's most dramatic phenomena unfolds.